The invention relates to a fluidised bed combustion apparatus in which in the free after-burning zone or the so-called free-board above the bed at least one system of plates or baffles inclined with respect to the vertical is arranged so that at least one upwardly directed channel having internally inclined surfaces is formed, such channel, seen in a vertical section in at least one direction, comprising at least one group of two opposed arrays of superimposed inclined plates. An apparatus of this kind is known from the article "Suppression of Particle Elutriation from a Fluidised Bed" in "Trans. Instn. Chem. Engrs", Vol. 52, 1974.
The application of a fluidised bed as a combustion apparatus has been recognized already for a long time as an attractive means for generating energy. In such a bed air is passed through a bed of particulate material consisting of a mixture of inert and combustible materials such as bituminous coal having a high sulfur content and usually an absorbent material mostly limestone, for absorbing sulfur dioxide which is formed as a result of the combustion of the coal. As a result of passing air through the bed said bed is fluidised whereby the combustion of the combustible material is promoted.
The advantages of such a fluidised bed are an outstanding heat transfer, a substantially uniform bed-temperature, a combustion at a relative low temperature, easy handling of the combustible materials, a decreased attack of and formation of deposits in the boiler and a decrease of the dimensions of said boiler and an improved control of the emission of harmful matter.
The quantity and the velocity of the air supplied to the bed must be sufficient for keeping the bed in a fluidised state. As a result a portion of the particulate bed material is entrained by the air which flows through the bed, which material contains among other things particles of the combustible material and of the material for bonding the sulphur which have not reacted. The air which has flown through the bed together with the gaseous combustion products and the particulate material entrained thereby flows in the free-board above the bed which forms a second reaction zone in which the partial combustion of the entrained solid particles and the gaseous combustion products is continued and wherein a partial reduction of the NOx formed in the bed and moreover a continued bonding of sulphur takes place.
After the free board the reaction gases and the entrained granular material are passed to a dust catcher for separating solid particles from the gases, which particles are subsequently discharged or passed to an apparatus which is not linked up with the fluidised bed. Because the granular material as discharged contains solid substances which have not reacted, the efficiency of the fluidized bed combustion apparatus is not optimal.
In order to increase the efficiency of a fluidised bed combustion apparatus, in said article is proposed to arrange in the free board a system of inclined plates or baffles by which system the particles entrained by the air and the combustion gases are for the greater part separated and returned to the fluidised bed. All plates or baffles extend from the outside to the inside in an upwardly inclined position whereas all plates are provided with an opening at the lowermost point adjacent the boundary wall so that the separated solid particles collected on the down stream face of a plate will slide downwardly along the plate and fall down through said opening. However this known arrangement is not efficient as there is the risk that the solid combustible material particles deposited on the plates or baffles will continue to burn and incrustations will be formed by caking. Moreover the solid particles will fall through said openings with difficulty as the upwardly directed flow rate of the air and the combustion gases is the utmost through these openings, for by these openings short-circuit flows between the zones of excess and reduced pressures at the bottom- and topside of a plate respectively will be created. Further, the combustion gases flowing out of the bed will hardly continue to burn, whereas no improved reduction of NOx and an improved continuation of the bonding of the sulphur will take place with respect to a free board without said baffles because the particles have been dropped out of the gas stream.
It is remarked that from U.S. Pat. No. 4,161,917 a thermodynamic generator having a fluidised bed combustion apparatus is known in which in the free-board a plurality of radial extending inclined vanes are arranged so that a rotational movement will be given to the combustion gases and the period of time during which the entrained particles remain in the free-board is extended, since these particles are thrown to the outside so that said particles are burned more completely. However, these particles are thrown against the wall of the free-board, after which no intensive contact between the solid matter and gases occurs anymore. Moreover, a restriction is provided above the vanes which can become clogged up.